When a machine cycle is to be repeatedly carried out with different tools, tool machines are programmed for operating a spindle through a working cycle such that the spindle will pick a chosen tool from a tool crib, will clamp its collet on the tool shank, will carry out the programmed operation on the workpiece and finally will return the tool to the tool crib. Two facing, annular registering surfaces on the spindle and the tool, respectively, axially abut with each other to insure that the tool is properly coupled with the spindle, so that a good degree of accuracy and repeatability is achieved.
However, minute chips may deposit on the registering surface during machining, which often stick to it, due to the capillarity of the cooling/lubricating fluid that is always present in the area. If the chip fragment is very small, of the order of a few hundredths of a millimeter (a few tens of microns) thickness, the compressed air usually blown over the spindle before picking the tool may be unable to remove it, and the chip will therefore be pinched between the abutted registering surfaces, thereby preventing the collet from closing, or, alternatively, causing a positioning error, generally involving a lack of coplanarity of the registering surfaces, whereby the tool is inclined with respect to the axis of the spindle.
It is known to provide the spindle head with a proximity sensor arranged for detecting the axial position of a control rod controlling the opening and closing of the collet, and for notifying a collet-clamping error in case of an incomplete stroke of the rod. However, such sensor is only able to notify coarse errors, due to the unavoidable variation in the nominal end of stroke of the control rod in different tools. Consequently, the above sensor is unable to detect very small fragments, which, however, as stated above, do cause a deviation of the tool axis from the spindle axis, with consequent linear errors amounting up to several tenths of a millimeter at the tool tip.